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1 Answer
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Syntactically, it is the copula だ with conjunct が, probably used in a cleft sentence with some omission. が often connects contrasting clauses, but as with this case, it can just connect two clauses that do not contrast. The literal translation is

'On top of the fact that (it) is my uncle who lives close-by (that I am going to talk about), I've heard that he isn't doing fine lately.

You probably unintentionally dropped the “who lives close-by” part from the translations.
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Tsuyoshi ItoJun 14 '12 at 11:58

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Just to add, the same usage exists for other conjuncts similar to が: 来月フランスに行くけど、お土産には何がほしい? does not really contrast two clauses, but the first part just sets up the context.
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Tsuyoshi ItoJun 14 '12 at 12:02